The wider definition of the écriture inclusive includes several writing changes.

I believe the main one the Académie is warning against is the reintroduction of a ancient sign (the interpunct) to coin words representing both genders or singular/plural, e.g.:

Les grand·e·s élect·eur·ice·s sont élu·e·s à la majorité des suffrages exprimés.

The issue is such sentences are extremely difficult to read aloud. Opposition to the écriture inclusive is not just from the Académie française. 32 linguists just published a paper listing several of its defects here.

The proximity agreement which your question focus on is also part of the loose écriture inclusive définition. It is arguably less controversial (but still advised against by the Académie française) as it doesn’t suffer the issue described above. It is in particular challenging the masculin l’emporte sur le féminin rule.

I have no statistics but given the fact it was already present in Latin, where all agreements were used (including using the neutral when an adjective was qualifying both masculine and feminine words, a convenience French cannot use), I guess this rule was widespread in older times.

Here is a web page presenting several examples of such proximity agreements like:

Ce peuple a le cœur et la bouche ouverte à vos louanges
« Il faudrait dit ouverts », explique Vaugelas en 1647. « Mais l’oreille a de la peine à s’y accommoder »

Ronsard, 1563 : afin que ta cause et la mienne soit connue de tous

However, it seems the proximity agreement found in Latin and ancient Greek was only applying to words representing things but not people. See L’accord de proximité en latin (et en grec) by Philippe Cibois.

A third older evolution is the introduction of new feminine forms or sometimes the reinstatement of old ones for occupation and similar names.

Some changes are now essentially accepted like une ministre, other are not (in France) like une écrivaine or une docteure. The Académie française already declared their opposition of some of these changes in 2002 including doubling masculine and feminine forms recommending les électeurs against les électeurs et les électrices and absolutely rule out a sentence like Le fauteuil et la table sont blanc(he)s which is similar but less disruptive than the middle point.