MMR

They removed it this season only because the various adjustments and resets to ranking that took place during the season made MMR calculations unreliable. They have indicated that it will return next season or possibly sooner.

incidentally, whatismymmr.com has been working all season. I use this site frequently, and their results are consistent.

because it was never accurate

Positional MM is probably at fault. Its likely more effort then is reasonable to attempt to track it... thou I have never played with Riots API so its possible I am completely wrong on that.

With old MM you could take results and put them into your own MMR system to generate an assumed value. As long as your class intervals are similar the exact points don't matter at all. Assuming that your using the same probability distribution model. If not your looking at a variance of a few % but still a workable way to make a ball park guess.

you can use these mmr checkers, they are pretty accurate: https://whatismymmr.tapnets.com/