Strictly: France will no longer decommission Belgium's nuclear power plants, as Belgium will buy them. The current owner Engie are majority-owned by the French government. Apparently there also used to be a phaseout policy which is being rescinded: https://www.msn.com/en-ca/news/other/belgium-and-czechia-ram... I'm not keen on new nuclear (time and cost as much as anything else), but it's a terrible idea to phase out…
I completely agree, but that's a massive "but". Belgium's nuclear power plants are mostly known for their reliability issues.
They are outdated 2nd-gen PWR reactors, designed by a company with no other nuclear experience, operating in some of the most densely populated areas of Europe. Keeping them operating long beyond their original design lifespan probably isn't the best idea - and it is almost a certainty that cleanup costs are going to be significantly higher than expected.
To me it sounds like Engie has struck an incredible deal by offloading a giant liability to the Belgian government.