What PDF standards are supported by iText (exactly version 5.5.10)? I can not find this information on iText web sites.
Why 5, why not 7? 5.5.10 is a maintenance release for people with legacy projects, not a release that new users should use.
iText 5 supports ISO 32000-1:2008 (PDF 1.7), but not ISO 32000-2:2017 (PDF 2.0). PDF 2.0 is only supported from 7.1 on, and PDF 2.0 support won't be backported to older versions of iText. The same goes with other new functionality such as the upcoming SVG support.
In digital signatures: iText 5 and 7 both support the PAdES standards, but the digital signature section has been completely rewritten for PDF 2.0, so you might want to use iText 7.1.
Both iText 5 and iText 7 support PDF/A (-1, -2, and -3). If you have iText 7 and the pdfInvoice add-on, you also have support for the ZUGFeRD standard (based on PDF/A-3 and the CII standard).
There's PDF/UA support in iText 5, but accessibility was significantly improved in iText 7, based on discussions on the ISO level (the section about Tagged PDF was completely rewritten for ISO 32000-2).
You'll see that there will be fewer and fewer iText 5 releases, so you should really consider iText 7. Another benefit is the improved HTML to PDF functionality: iText 7 was rewritten completely from scratch to have a better match between HTML, the iText syntax, and the Tagged PDF syntax.
We didn't invest in PDF/X or PDF/VT yet (no paying customer is interested in those standards). We probably won't invest in PDF/E because PDF/E-2 was abandoned by ISO: there will be no PDF/E-2 in favor of PDF/A-4 which will have engineering topics.