Currently the validation of XML names is based on the original 1998 specification which defines a large set of codepoint ranges that are to be accepted. These ranges were widened and simplified in the fifth edition of the spec, published in 2008 and now the current version.

The name production rules are now:

NameStartChar  ::=       ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | 
                           [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | 
                           [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | 
                           [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | 
                           [#x10000-#xEFFFF]
NameChar       ::=       NameStartChar | "-" | "." | [0-9] | #xB7 | 
                           [#x0300-#x036F] | [#x203F-#x2040]
Name           ::=       NameStartChar (NameChar)*
Names          ::=       Name (#x20 Name)*
Nmtoken        ::=       (NameChar)+
Nmtokens       ::=       Nmtoken (#x20 Nmtoken)*

This may also address the majority of the requirements for xml1.1 support (#25755) since the changes between 1.0 and 1.1 were the expansion of the name character ranges, the addition of two line ending characters (U+0085, U+2028) and specification of additional normalisation rules

The current ranges span 300 lines of code in the xml package so changing this will also contribute to #26775

If there is interest then I can submit a CL.