In traditional mode ($wgExperimentalHtmlIds = false) Sanitizer::escapeId generates ambiguous values:
Sanitizer::escapeId( '!', [ 'noninitial' ] ) and Sanitizer::escapeId( '.21', [ 'noninitial' ] ) evaluates to .21.

A heuristic decoding would generate relevant errors. For example the heading

172.31.255.255

gets encoded to id="172.31.255.255" and a heuristic decoder would generate 1721%5%5.

It would be possible to generate nonambiguous values by encoding . with .2E. Sanitizer::escapeId( '.21', [ 'noninitial' ] ) would evaluate to .2E21.

Such a change would be easy to implement by adding

'.' => '%2E',

after line https://phabricator.wikimedia.org/source/mediawiki/browse/master/includes/Sanitizer.php;d0a0838cb76b4cf20977c4aba5fe06877d8deb58$1205

But such a change is not backward compatible. Existing anchor links to headings can break.