I work for Peugeot/citroen due to block exception servicing outside dealer group will not invalidate the warranty… BUT!!!
You have it service by an independent and the Ecu has a fault that causes the particle filter to not regenerate for instance… And they don’t notice/fix it/send you to a dealer for it… And the particle filter dies… £££ I could look into the Ecu historically and find at the service the Ecu fault codes wasn’t read. Then your warrenty is invalid because you haven’t carried out the servicing properly because the garage hasn’t carried out the SERVICE MANDATORY operations.
Also say the turbo dies a few months outside warrenty… The first thing they look at it’s full dealer history. If not that’s the very first reason for refusal.
Nothing wrong with indipendents, some fanatic ones out there, but they all tell you it won’t cause warrenty issues but I deal with problems like this daily.