What about letter fonts? Are they libre?

The Liberation fonts are open source and licensed under the SIL Open Font License.

most fonts are very much not free, but in a "rights" sense rather than in a "standards" sense. since fonts are works of art rather than technical designs, open source licenses don't really apply. when printing a book you typically select a font and buy a license for that font that only allows you to use it, not modify it.

in fact, most of the web is built with proprietary fonts that keep being used in spite of their licenses because the alternative would break everything.

I was reading into this recently too. My understanding is that Debian + LibreOffice install the Liberation Fonts by default so usually you don't need to worry about documents using Ariel and Helvetica (Liberation are metrically compatible replacements).

After further reading I ended up also installing the Crosextra fonts, the advantage of those two is that they are metrically compatible with Microsoft's Calibri and Cambria. Once installed in theory LibreOffice should be able to open documents that had those Microsoft fonts and auto replace them with the open versions. (there's a setting in LibreOffice to force font replacements but it didn't seem like I needed to do that in my case)

Some more info on that

https://en.wikipedia.org/wiki/Croscore_fonts#Crosextra_fonts

The Crosextra font packages in Debian

https://packages.debian.org/trixie/fonts-crosextra-carlito

https://packages.debian.org/trixie/fonts-crosextra-caladea

Also interestingly - if you do really, really want some of Microsoft's fonts they are free to install but I don't think you actually get a license to distribute/publish with them. I didn't bother installing these but could be useful for someone with tons of old MS Office documents with lots of random MS fonts.

https://packages.debian.org/trixie/ttf-mscorefonts-installer (need to enable contrib in your apt settings to be able to apt install those)

They can be. Google Fonts is a good source for freely licensed fonts.

https://en.wikipedia.org/wiki/Intellectual_property_protection_of_typefaces

SUSE released a set of completely open source sans serif font for targeted at os-level fonts. It is a pretty good font and I started using it on many of my systems.

Fonts do have patents and copywrite laws as "works of art" or in the methods to convert what you see on a computer to your printer or whatnot. For example here's the history of Calibri:

The Typography Group at Microsoft is responsible for both fonts and the font rendering systems in Windows. Since version 3.1 the primary font system built into Windows has been the TrueType system, licensed from Apple in a deal (with hindsight) remarkably beneficial to Microsoft. Working with Monotype, the Microsoft Typography Group produced fine TrueType versions of Arial, Times New Roman and Courier New, tuned to be extremely legible on the screen; these were all ready for the launch of Windows 3.1. Since then these core fonts have been developed to cover more and more of the world’s languages. In the mid-1990s under Robert Norton a program of truly new type designs was begun, using TrueType technology to render faithfully the bitmaps and outlines designed by Matthew Carter (Verdana, Georgia, Tahoma) and by in-house designer Vincent Connare (Trebuchet, Comic Sans). Until August 2002 these “core fonts” were offered freely over the Web, where they made an undoubtedly positive contribution in terms of legibility and font choice. In 1996 the OpenType initiative with Adobe was announced; this is touted as the end of the font wars’, whereby advanced multilingual text layout becomes available, native rendering of PostScript fonts becomes part of Windows 2000, and unwieldy font formats are rationalized. In 1998 the group announced ClearType. This is a very ingenious method to increase legibility on color LCD screens, individually targeting the 3 subpixels (red, green and blue) that make up each pixel. Such a leap forward in readability on these screens is a crucial element to the success of nascent eBook technology. Simon Daniels at the Group’s website keeps font fans and font developers up to date with most aspects of the digital typography scene, and communicates the technicalities of how fonts work in Windows. Updating us about the current (October 2000) activity of the Group, Simon notes: 1999 saw several members of the group leave to join Microsoft’s eBooks group. These included technical lead Greg Hitchcock, developers Beat Stamm and Paul Linerud as well as former Monotype hinters Michael Duggan and Geraldine Wade. The past twelve months has beeen a rebuilding period for the group, with numerous new hires [sic.] replacing earlier departures. The Group continues to provide font related services for Microsoft, and freely licensed tools and technology to the wider type development community. On August 12, 2002 Microsoft discontinued the free availability of the “core fonts”, noting that “the downloads were being abused” in terms of their end-user license agreements. Most commentators took this to mean the company objected to the fact that the fonts were being installed with Linux distributions.

https://www.myfonts.com/collections/calibri-ms-font-microsoft-corporation

A friendly heads-up: I think you mean "Liberation Serif". Serif and sans-serif ("without serif") are common categories for fonts.