I have written a height based reference implementation as well as updated the BIP text in the following proposals
"lockinontimeout" was just an implementation detail to allow BIP8 the BIP9 implementation code. With the change to height based, we can dispense with it entirely.
So the two changes BIP8 brings is BIP9 modified to use height not time, and remove the veto failed state.
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Subject: [bitcoin-dev] Height based vs block time based thresholds
Some people have criticized BIP9's blocktime based thresholds arguing they are confusing (the first retarget after threshold). It is also vulnerable to miners fiddling with timestamps in a way that could prevent or delay activation - for example by only advancing the block timestamp by 1 second you would never meet the threshold (although this would come a the penalty of hiking the difficulty dramatically).
On the other hand, the exact date of a height based thresholds is hard to predict a long time in advance due to difficulty fluctuations. However, there is certainty at a given block height and it's easy to monitor.
If there is sufficient interest, I would be happy to amend BIP8 to be height based. I originally omitted height based thresholds in the interests of simplicity of review - but now that the proposal has been widely reviewed it would be a trivial amendment.