Can I delete these old logs in my /data dir?
Generally speaking, nothing in the /data subdirectory of a SingleStore installation should ever be manually modified or deleted unless a SingleStore support engineer gives specific instructions to do so. This is because transaction logs in the data directory are not end-user human-readable logfiles; instead, they are a record of inserts/updates/deletes to the database itself, and their deletion will result in data loss.
The logfiles in /tracelogs, on the other hand, are indeed end-user log files and may be safely deleted if they become large enough to cause concern about disk space utilization. The tracelog file will consistently be named *memsql.log by default.