It is incredibly simple.

By adding this

-steamMM

to your Payday 2 Launch Options (Accessible by right clicking the game in your library, then click properties), you will return to Steam Matchmaking. If you have added previous launch options before, then you know how this works and how to put it, but incase you haven't and you have other launch options in your text field, just put it at the end.

That's it. That's all you have to do.
It is weird to me that Overkill didn't implement it as a toggleable option ingame under the name Crossplay or something similar, probably because users on EGL would be left in the dust on the platform.

This DOES LET YOU return to the Steam Matchmaker, but keep in mind, as this was JUST DISCOVERED, not everyone knows about this. This means in essentiality that only a few people have this option turned on. This does fix issues with crime.net such as it not working and heists flickering on and off rapidly, while some issues still remain. For this to work better and better, spread the guide around or manually tell people HOW TO RE-ENABLE STEAM MATCHMAKING ASWELL!!!. The more people on the steam matchmaker, the more heists will show up on crime.net, and the faster quickplay or looking for random teams will become.

Source for this finding. Made this guide just so more people can know.