FWIW, while I think overall this is a positive change, I think it is not backwards compatible, as it seems to replace SimpleDateFormat with DateTimeFormatter, which produces different output for the same formats.

for one example, see https://stackoverflow.com/questions/36722873/datetimeformatter-not-compatible-with-simpledateformat
for another, using the date represented by 2018-01-08 12:30:05 -0800, the format for the day of week in month - {{dateFormat date format="F" tz="UTC"}} - used to produce 2, but with this change, it produces 1

Thanks for pointing this out, I was not aware of this.
These seam to be the patterns that changed in meaning completely, aside from potential formatting differences:

Edit: DateTimeFormatter F seams to change definition between Java 8 & 11 from week-of-month to day-of-week-in-month thus matching SimpleDateFormat. I did not test any of this, just reading the linked docs.