Through the Section 508 regulations, the US Access Board sets accessibility policy for US federal agencies and contractor's deliverables.
"[Essentially], PDF/UA is required (as an option) for software that can create *modern and good* PDFs," said Bruce Bailey, Accessibility IT specialist at the US Access Board.
The verbatim requirement is as follows:
504.2.2 PDF Export. Authoring tools capable of exporting PDF files that conform to ISO 32000-1:2008 (PDF 1.7) shall also be capable of exporting PDF files that conform to ANSI/AIIM/ISO 14289-1:2016 (PDF/UA-1) (incorporated by reference, see 702.3.1).
(Source: Federal Register)
The original ISO standard costs $111, and is available directly from the PDF Association.
The PDF Association publishes a variety of materials intended to assist developers and users understand PDF/UA, including:
Bailey acknowledges that the specification of PDF 1.7 in the regulation's text allows vendors who want to get by with restricting themselves to earlier versions of PDF can evade the PDF/UA requirement. He notes, however, that "...we would not expect that authoring tools compete on their lack of features."
Unfortunately, IBM is one such.