Absolutely not. They are trying to protect you, can you imagine how awful Safari would be if it let you sideload such nasty extensions? It would be just like Chrome, absolutely no market variation to speak of! Despicable.

It's really AdGuard's fault for failing to fit their functionality within the arbitrary constraints Apple decided was suitable for a runtime.

It is not an arbitrary decision.

For one Safari compiles block lists to perform better, but it can be noticed at startup for big lists.

Then there is just resource constraints since the focus is mobile. Chrome on mobile notably supports no extensions.

But I do wish desktop Safari was more lenient.

Chrome's decision is also entirely arbitrary, so it's not a great example. Firefox on mobile notable supports Chrome extensions, without any real issues or battery drain whatsoever.