The French tax authorities posted a publication with Frequently Asked Questions regarding the Brexit. UK VAT ID numbers can indeed no longer be checked in the VIES database (which can be found here: https://ec.europa.eu/taxation_customs/vies/?locale=en).
Instead, if you want to check a UK VAT number, you can do so on the website of HMRC here: https://www.tax.service.gov.uk/check-vat-number/enter-vat-details
Source impots.gouv.fr