Hopefully you know the Man in Seat 61 is the knower of all knowable train knowledge. He has a very complete COVID page on his site listing the current situation in every European country (albeit British-focused).
I noticed he had cooked up two possible train options for Versaille:
"The famous Chateau at Versailles (www.chateauversailles.fr) is 30-40 minutes southwest of central Paris using a frequent express metro train on RER line C to Versailles-Rive Gauche station. Versailles-Rive Gauche is 5 minutes' walk from the entrance to the Chateau. RER line C runs directly to Versailles Chateau - Rive Gauche from various RER/metro stations in central Paris, including the Gare d'Austerlitz, St Michel-Notre Dame, Musée d'Orsay, Invalides, & Champ de Mars-Tour Eiffel. From the Gare du Nord, take RER line B two stops to St Michel-Notre Dame, then change onto RER line C to Versailles-Rive Gauche.
A nicer way to reach Versailles from central Paris is to go to the Gare St Lazare and take Transilien line L to Versailles-Rive Droite. These Transilien suburban trains run every 15 minutes from platforms 1-4 at the Gare St Lazare, taking about the same time as the RER C route, but unlike RER C almost all the journey is above ground with views of wealthy Parisian suburbs, a Seine crossing, and the towers of the financial district at La Défense. You arrive at Versailles-Rive Droite which is a slightly longer walk from the chateau, but you walk through the old part of town and can enter the chateau through its gardens at the 'Bassin de Neptune' rather than through the car park as with the walk from Versailles-Rive Gauche."