You could wrap the string under question in parentheses. createdParts[k]:IsA(("DeallocatedPart")). This is because the linter is pattern matching for these expressions, but sometimes it has false positives like this. We want to improve it to be more type aware.
Alternatively, you can disable this particular lint pass in whole by writing --!nolint UnknownType on the top of the script, but that’s going to disable potentially real issues. It’s documented in Linting - Luau.