Generally, for a "normal" mechanical keyboard, just pay a few bucks extra to make sure it claims to have QMK and/or one of VIA or VIAL. The first is an open source firmware that, assuming the maker followed the license properly and released their code, allows for accessible updates and reflashing. The other two are actually WYSIWYG web/electron apps that run on top of specially configured QMK installs and allow you to change keymaps/macros/etc. on the fly without re-flashing. Some keyboard makers (almost exclusively Chinese companies at this point) are better citizens than others about their open source software, but for simple functionality you want to make sure they're advertising that they use them.