MDL-71468 removed $CFG->conversionattemptlimit since it removed the assignfeedback_editpdf_queue table - which was responsible for keeping track of how many times a conversion had been attempted.

Now that conversions are handled by ad-hoc tasks, it's possible for a task to get stuck indefinitely when a problematic PDF which can't be converted is uploaded for example. Adhoc tasks do have a fail-delay mechanism, which has an upper limit of 24 hours. Meaning that conversions that fail in non-resolvable ways will be retried every 24 hours forever.

We could re-implement this by making it part of the file converter code instead of being specific to assignfeedback_editpdf. One way to do so would be to add a column to mdl_file_conversion which tracks how many times the conversion has been polled.

Another option is to re-introduce the assignfeedback_editpdf_queue table, as well as a scheduled task that uses it (similar to the old scheduled task pre-MDL-71468) - this scheduled task would then be responsible for spawning the adhoc tasks to poll conversions (the tasks would have to succeed even if there is an error, otherwise they will be retried over and over again as mentioned above).