Making your project browser backward compatible is a big mistake.
They will keep old software running with all security issues that this involve
They will spend a lot of time and money trying to make something work for a few number of users that does not care about secutity and updates (remember WannaCry and all those XP os running after product EOL ?)
If you accustom to people that even in a IE8 internet works then, when something does not work they will ask the corresponding IT department complaining about why it does not work, spending a lot of human resources and money.