I'm going to disagree here a bit. Now I come form the days when we mixed 1e & BECMI because we didn't realize they were different. However, my options we also extremely limited to what was at my local game store. With 5e and the internet (and decades of experience) I have such a huge range of options with 5e. WotC has not only put out more information than I had access to "back-in-the-day," but there is 10 or maybe closer to 100 x the information available from homebrews at my finger tips on the DMsGuild or the UA Reddit or here.I can't speak of 5e with a lot of confidence. While there is a lot of talk of "rulings not rules", my suspicion is that the tables nearly as monocultural as 4e with respect to mechanics, as there is simply not a lot of material out there, and 5e I think so far has very much focused on a single very popular style of play and appeals mainly to groups that play in that manner. I see nothing like the recognition that a system that works well for combat might not work so well for chases/evasion, mass combat, or what have you and I don't think that many 5e tables have yet focused on dynastic play or any of the other sorts of things that 5e simply hasn't mentioned much in the thus far published rules. However, that's all just a guess.
5e, and the 5e community, appear to embrace the idea of mix and mashing other system with and into 5e. For example, I have seen multiple 5e rules for: epic play (beyond level 20), mass combat, the warlord, ranger, well about any class really, variant death and recover rules (for that 1e feel), weapon speed, alternate initiative, classless characters, spells, magic items, armor as DR, morale, insanity, chases, strongholds, retainer, etc. The 5e community is overrun with mish-mashers IMO. There are many of these in "official" UA or provide by the designers themselves, so there appears to be official support for this behavior as well.
Now, how many people use these systems I don't know. But many people present them as things they are using in their game. If the idea of mixing systems died in 3e, I would say it has been reborn with 5e.
EDIT: I just checked. I have downloaded 204 rules variant documents from the DMsGuild, UA Reddit, or similiar for 5e. Not counting races, monsters, and equipment which would had a few hundred more. That is more than my entire hard copy library of 1e, BECMI, 2e, 3e (I didn't play 3e so only one product here), 4e, and 5e combined. Many of these variants are bringing ideas from older D&D editions or other systems to 5e. I think we are possibly in the Golden Age of system mixing actually.