You can tell your clients you only support whatever you want, and they can tell you they won't be a client of yours anymore unless you support what they want.
Theodores: I only want to support IE11+
Client: No. What if our users have older browsers?
* later *
Theodores: I only want to support IE11+
Client: No. What if our users have older browsers?
Theodores: That should be exceedingly rare, Microsoft have actually pulled support for those versions recently. See this link.
Client: Oh, ok then, great. IE11+ it is.