Postgresql query slowed with table growth
You are assuming that the problem is the switch to the bitmap scan.  That is very unlikely.

The likely problem is that your data had to be read from disk, either because it has grown too large to fit in cache (in which case more RAM would help), or the cache was cold when you happened to run the test.

The switch to bitmap scan is likely a response to the same thing that caused the problem, the growth in data.

You could cluster the table on the index t_feature_ind, which would make it more efficient to pull out all of the data with any given feature.