Is doing good deeds with the intention that people remember one and supplicate for them after death considered showing off (riya’)?

If a deed is purely for the sake of Allah, then the intention for people to remember you and supplicate for you after death does not affect the validity of the intention and is not considered ostentation (riya'). This is because it is a good intention that does not contradict sincerity. Ostentation is performing a deed for people to see you, seeking their elevated regard, or seeking a position in people's hearts by displaying good qualities, or desiring people's praise through obedience to Allah.