We're running elasticsearch version 6.8.8 and 7.7. In both clusters we observed the bahaviour, that the cluster is setting the parameter read_only_allow_delete to true as soon as the fllod stage is reached for some of the indices. The reason is clear to us: During merging the index segments the disk space went near 100% and therefore the parameter was automatically set to true.
BUT: Is there any reason why after merging is done and disk space is (at least 85%) free again the parameter is not set to false automatically?