I have no real experience with C/C++ as well as game development but I always wonder what would happen if old games like these would get recompiled with an, for example, avx2 flag.

Very little. Quite a few programs recieves a decent boost when compilers moved from a 386 default to 486, then Pentium 1. Since then the gains have been much smaller, unless you use the instructions on purpose.

Does someone know if IE11 will get brotli support? Or will IE11 only receive security patches?

Security patches only. IE11 exists on Windows 10 only for the benefit of enterprises that can't drop horrible legacy technologies like ActiveX. All new feature development is on Edge.

I am waiting for this as well.