Is storing old, outdated lead data in our database causes GDPR non-compliance?

I just read the following in a guide from Synthio:

Outdated data, misinformation, duplicates, and data input errors used to be merely an inconvenience or a mild annoyance; but with the GDPR in place, these inaccuracies can result in non-compliance (and the gargantuan fines that accompany non-compliance).

Is this true? If yes, how are others handling this in Marketo? How much should we wait before we mark a record as outdated, and then decide if we need to delete it?

TIA

Rachit