Pun intended? But seriously, whenever I see one of these browser-blockers (and it seems Opera gets hit too), I just leave the site and never come back. Chances are, your site is not unique and I'll find what I need somewhere else.

developers can spend more time developing features

IMHO, those are "features" which no one really cares about. Stop messing around with fancy CSS and JS, spend the time developing a simple site that almost naturally works in IE too.

I agree that targeting of Edge browsers is ridiculous since it's keep current, on the other hand for IE and if the site is non-commercial/personal (commercial sites should really try to be compatible on as many browsers as they can) I don't really blame them if they're wanting to use modern web standards. Microsoft has abandoned feature updates to IE, and sadly only made Edge available to W10 users.

Flexbox in CSS is one example. It was made to simplify the creation of page layouts and content flow and includes various features that were sought-after for a long-time that require hacky or more complex solutions otherwise. The majority of its bugs, or just plain missing features are found in IE10-11 (and absent completely in earlier versions). Either creators design the site to entirely compensate for IE and loose that flexibility (pun not intended), find some Javascript polyfill that actually works/fixes such bugs, or just present some message to users for unsupported browsers (not that it needs to necessarily block the page though). It's just one of several useful features that make things far easier but lacks full/partial/any support in IE.