Welcome to Byzcath, ProudByz!
A while ago we had a poll about how many parishes use Old Slavonic. Sadly the numbers appear to be rather low.
To see a thread on the subject of Slavonic usage:
https://www.byzcath.org/cgibin/ultimatebb.cgi?ubb=get_topic;f=8;t=001786#000001
I wish there was at least permission to use Slavonic more often in the liturgy. I know the push for vernacular started some 50 years ago, followed by the approved 1965 translation and music followed now 40 years later with new revisions to the translation, resetting the new translation to the original music rather than simplifying it. With all that, one might hope that the Slavonic would have survived at least
partly used, but alas, no, it was pushed out of the way as too 'old world'.
I feel that there is still a place for Slavonic in Liturgical use; while the vernacular should be emphasised, the Slavonic shouldn't be put to pasture. <sigh>
Steve