IE 11 is still supported by Microsoft because it is tied to Windows 10 support.
If you're B2C you can probably ignore that and just not support it, but if you're B2B where you're selling not to users it's very hard to get away from supporting IE11.
If your users never directly interact with you (for example you sell white-label software which gets resold) then you just can't control your end-user tech stack enough.
If you're selling to partners who sell to companies who push out logins to their customers or user base then even if <1% of users use IE 11, that becomes 5% of companies having a user with it, which becomes 30% of the partners who are asking for IE11 support.
It's one thing to turn down 1% of users it's quite another to annoy 30% of your income stream.
As long as bootstrap 4 is supported (and the legacy bootstrap 3 support suggests it will be) then this doesn't have to be a problem of course, just one more thing to be aware of.
If you're B2C you can probably ignore that and just not support it, but if you're B2B where you're selling not to users it's very hard to get away from supporting IE11.
If your users never directly interact with you (for example you sell white-label software which gets resold) then you just can't control your end-user tech stack enough.
If you're selling to partners who sell to companies who push out logins to their customers or user base then even if <1% of users use IE 11, that becomes 5% of companies having a user with it, which becomes 30% of the partners who are asking for IE11 support.
It's one thing to turn down 1% of users it's quite another to annoy 30% of your income stream.
As long as bootstrap 4 is supported (and the legacy bootstrap 3 support suggests it will be) then this doesn't have to be a problem of course, just one more thing to be aware of.