And it will be only worse. It's unrealistic to support all browsers, they will always have to draw the line somewhere, e.g. it's obvious that sites can't work even in all historic browsers. But those aside, you can perhaps say that it should be all maintained browsers, that probably sounds right. But what if, for any reason, some maintained browser doesn't support something that others do?
Let's say it's deliberate and authors decided to boycott something. Should webmasters and all other browsers just give up? The answer should be probably "no", otherwise it would be possible to halt all progress this way. It should be that browser's problem. Or maybe it's that some browser just lags behind and needs more time to implement things. Then it would be nice to wait for it. But for how long? Year, two, five, more? Surely, at some point, difference between the two (boycott / lag) vanishes, because it's the result that matters.
In the end, it all depends on who you are and what power you have. If Pale Moon doesn't implement something (Web Components, all new JavaScript stuff) fast enough, it's easy to throw it overboard, because it means losing at most 0,0x% visitors (even much less, most will come back with another browser), no big deal. But if Chrome decides to boycott something (
JPEG-XL), there's no way webmasters are throwing Chrome overboard. Quite the opposite, most will simply not use JPEG-XL at all, because they would have to create another/modified Chrome-compatible website anyway, so it's easier to create only that one and it will work for others too.