Mustangproject version 2.9.0 was released today, November 27th, 2023.
New features include

• Previously one had to specify if the input was PDF/A-1 or PDF/A-3, now with #341 we have input PDF-A Version autodetection
• It can now validate XRechnung 3.0.1 #347

and it comes with the following fixes

• Missing closing tag in BankDetails when there’s no BIC number #339
• PDF/A-3 input now also sets a default ZUGFeRD version

The following known issues remain:

• the Xrechnung validation still only covers the CII version #337,
• it does not yet write XR 3.0 #343 and
• there are still known ZUGFeRD upstream issues with validation in particular affecting XRechnung #316