Are pibase extensions still working in TYPO3 8?

I have to migrate TYPO3 6.2 websites to 8.7. Some websites use custom pibase extensions, do I need to redevelop them with Extbase ?

you don't need to redevelop these extensions, but you might need to change the call to core functions.
In 6.2 you still could use the old class names like t3lib.
These class names are available only with compatibility layer (together with a lot of delay).

For the future you need to use namespaces (and the correct new classes). You also should use namespaces for your own classes.

Depending on your used functions you might need to replace some calls with the newer functions as some functions got deprecated meanwhile.