I’ve been wondering whether these is a feature like this available in cAP ac’s. I’ve seen similar features in APs from different manufacturers.
Decrease TX power of 2,4GHz, and cross the fingers because all are depending on software used on smartphone.
And one day maybe with band steering (ros v7?)
I do not know if is added or not on future 
How do you measure thet 2.4GHz “gets crowded”? Number of devices? Traffic? Signal quality?
You can write a script which counts devices connected to 2.4 bands and then change ACL to drop block new connections or drop all these with quality below limits to “persuade” them to reconnect to better quality band.
I have the impression that the missing information in MT does not allow to estimate if it is getting crowded or not. Other brands clearly use this info for AP and band steering. The client decides (as always in wifi) but is given (QBSS load element) some information to help in that decision. With MT I did not find this in the beacon. But even within ROS I did not find that information, unless stopping the AP and doing “Freq Usage” scan. The process of CCA (clear channel assessment) happens before every transmit, so ROS could know, but that information is not at our disposal AFAIK.
http://forum.mikrotik.com/t/802-11ac-severe-speed-degradation-with-ros-above-6-45-9-lts/148854/1
I used to disable 2.4 for a few seconds every 30 minutes.
Dual band devices would see that 5Ghz was available and usually jump.
An idea I played with, was trying to make an ACL based on radios that had connected to 5Ghz. This was the overly simple stupid version of what the Ruckus ZD used to do (well one of the things). It would occasionally boot these units off the 2.4radio, if on one.
Then they made radios generate fake Mac addresses… And I gave up on that.
Just wondering … could something be done with “Interworking Profiles” , to inform clients that they are not welcome at a certain busy point in time ?
https://wiki.mikrotik.com/wiki/Manual:Interworking_Profiles
Never tried that one … .
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Might need some faking and abusing Hotspot 2.0 features, and needs compatible devices .
https://mum.mikrotik.com/presentations/ID16/presentation_3271_1476849522.pdf
Why script it, wlan interface has Max station Count option so you can limit number of devices there, other vendors do this by default as i saw on some (Aruba for ex)
Yes you can restrict the number of clients. But it is for me by no means a measure for the load on that AP. 100 clients that are quiet give almost no load (happens more these days than some time ago with smartphones, smartwatches etc). 1 client, from far away, transmitting or receiving at 6 Mbps, can be an enormous load (consumption of air-time) even for a limited amount of data.
Well what can you do about this client?If hes that far, he will probable have worse signal or no signal on 5ghz..
How do you even know client has 5ghz capability?You will end up endlessly kick him from 2.4 without ability to connect at all..
Theres 1000 more ifs i could come up…