Another thing to consider is that PHP developers have a track record of deprecating certain functionality, then a few releases later changing their minds and no longer deprecating it. Some examples of this include is_a, and var in property declarations.
Given how it's still relatively common to see PHP 4.x installations in use today, 10 or more years after they were first released in some cases, merely deprecating some of the broken functionality now won't help much. We'll still see deprecated features in use in 2020, if not well beyond then.
The broken functionality needs to be stripped out completely within a reasonable time frame (well under a year), not merely deprecated and left around for years, if it even stays deprecated. But this involves the PHP community acting responsibly, and going along with these changes for their own good. I don't think we can expect that level of responsibility out of them, unfortunately.