I don't think the SegmentMerger should uncompress automatically? That'd make <=2.9 indexes suddenly bigger.
If we re-add support for compressed fields, we have to also provide the special FieldForMerge again. To get rid of this code (which is the source of the problems behind the whole COMPRESS problem), we could simply let it as it is now without FieldForMerge. If you merge two segmets with compressed data then, without FieldToMerge it gets automatically decompressed and written in uncompressed variant to the new segment. As compressed fields are no longer supported, this is the correct behaviour. During merging, the compress bit must be removed.
The problem are suddenly bigger indexes, but we should note this in docs: "As compressed fields are no longer supported, during mering the compression is removed. If you want to compress your fields, do it yourself and store it as binary stored field."
Just for confirmation:
I have some indexes with compress enabled (for some of the documents, since 2.9 we do not compress anymore, newly added docs have no compression anymore [it was never an good idea because of performance]). So I have no possibility to get these fields anymore, because I do not know if they are compressed and cannot do the decompression myself. For me, this data is simply lost.
I think Solr will have the same problem.