This restriction is still in place as there is no TLS decryption on the MX/Z-series. Two years ago there was a CiscoLive presentation where it was stated that it is coming, and there already was a screenshot showing it. But I wouldn't expect that Meraki will really implement it. My guess is that they just rely on Umbrella SIG for HTTPS decryption.
I am not aware of any changes in checked file sizes. But with only HTTP inspected, AMP is not really relevant nowadays.
For the TLS decryption: Yes, beta (or alpha?), and I would expect that Meraki will drop this feature in favour of the Umbrella SIG integration. But that is only a guess. And I don't want to imagine what happens to my MX64 after enabling the inspection. Probably it will just burn and cry because of missing resources ... 😉
I have foudn AMP to be a pain and it has caused me numerous issues with online platforms like Steam where it was obstructing downloads for no reason with nothing listed in the event logs as to why traffic was blocked.
Meraki CMNO, Ruckus WISE, Sonicwall CSSA, Allied Telesis CASE & CAI