do we have statistics on how many de minimus votes have occurred on POB
I have not pulled that info, but it could be done.
does ratification of a proposal mean it can't be altered in the future?
A ratified proposal could be altered at any time, by whatever governance protocol happens to be in place at the time. At present, the stated governance protocol requires greater than 50% of stake-weighted POB voting affirmative.
As @mineopoly pointed out in a separate comment, that means abstentions are effectively "No" votes. Personally, I would rather see a higher threshold (e.g. 67% instead of 50%), but have it be based on whomever votes within the stated timeframe.