Done in automake 1.13.2:
------------------------

I have discouraged the use of '.txi' and '.texinfo' suffixes for
Texinfo inputs (see commit 'v1.13.1-6-ge1ed314') and the generation
of suffix-less info files (commit 'v1.13.1-4-g2af418d').

This is done to ease transition to Automake-NG (see commits
'v1.12.1-416-gd5459b9' and 'v1.12.1-392-ga0c7b6a' there), and
(to a lesser degree) to discourage use of seldom-tested setups.


The Future:
-----------

I have no plans to do the further step of removing support for those
usages in future Automake versions.  That would be a gratuitous
incompatibility (in Automake-NG, they have been useful because have
allowed further refactorings and improvements, but those were
based on GNU make features, and as such have no place in mainline
Automake).