New Trisquel user

> Why some well-known free software with a GPL license is not available in repos?

Probably nobody packaged it. Trisquel bases on Ubuntu which bases on Debian. If neither of these 3 had resources/incentives to package something, it shall be missing.

Some very popular programs are hard to package because of numerous dependencies that need to be packaged first. This is especially true for many systems in Debian family where creating a high-quality package that meets distro's repo standards takes a lot of work.

Debian, Trisquel & friends are mostly LTS distros. Because of that some programs like cryptocurrency blockchain node implementations make little sense to package because they would become obsolete (by scheduled blockchain forks) long before the distro release exits stable.

> What would happen if I install trisquel on my old i3 Lenovo Ideapad laptop will it be able to boot?

What makes you think an i3 laptop is old? Trisquel runs fine on ThinkPads from 2008.

In general, it is a good idea to first boot a live USB and verify that everything (audio, OpenGL, LAN, suspend&resume, etc.) works as expected. Then you can proceed with installation

>"Why some well-known free software with a GPL license is not available in repos?"

I agree with all that @koszkonutek said. Also, if you want more GPL licensed packages, you can install the fully libre Guix package manager on Trisquel and have access to over 20,000 up-to-date packages. I wrote a how-to on how I installed and used Guix on Trisquel here:
https://trisquel.info/en/forum/installing-guix-trisquel-10-w-luakit-and-ungoogled-chromium

"/// Why some well-known free software with a GPL license is not available in repos? ///"

I don't think any distro has ever had every package I wanted. I have had to use Flatpak's; PPAs; App Images; etc on Trisquel but I also did on Manjaro and Garuda as well. You can't please everyone as they say.

"/// What would happen if I install trisquel on my old i3 Lenovo Ideapad laptop will it be able to boot? ///"

It should boot. Trisquel has worked well with various systems in my experience. I have it running on hardware as old as a T61 and as new as a Intel i7-6700 desktop as well as various AMD systems in between. If there is one thing that would give you issues it would probably be the wifi but it would still boot.