Icon fonts were always a workaround for vector graphics, this because SVGs were not well supported. So good to hear newer browsers have better support. Unfortunately, one does generally need to support users on older browsers.
https://caniuse.com/svg https://caniuse.com/svg-img
It seems to me the only browsers not supporting any SVG are ones you'd expect in a computer museum, but not in active use.
The flipside is that one just accept that the users who are not able to upgrade are not your target audience and will use other systems to get their work done.
Don't carry others burdens when they self impose their problems.
Or in healthcare.
Or in industrial systems.
Or in kiosks.
Or in much of the real world beyond social media and "disruptive" industries.A healthcare provider using a browser that's too old to have SVG support is a patient data leak waiting to happen.
[1] https://css-tricks.com/a-complete-guide-to-svg-fallbacks/
How about security?
That's not changing any time soon; even with old IE ripped out of Windows 10 to a large extent, there are still multitudes of kiosks and other custom setups using XP/Vista out there that won't be getting any upgrade.
Security should absolutely be more of a concern but it's not. You're talking about business choices not engineering choices.
I think the plan is for Edge to one day have a way to display pages in IE compatibility mode.
Developers almost never get to decide if they want to give up on IE, unless they're doing their own project. It's always the legal, security, and other departments and middle managers that make that decision.
I can imagine the response of a developer at an enterprise saying to the boss, "I'm going to stop supporting 2% of our customers, is that OK with you?"
Depends on whether it's followed by "This will save 20-50% of development and maintenance costs."