I cannot speak to GoDaddy's practices. However, I can say that for Namecheap, this is not something we would ever even consider doing.
In my experience though, lookups are more complex than most think. We are querying so many different sources to give you availability status, some of which are less reliable than others. For example, with smaller TLDs like .ai or .is, lookups may be less reliable than a well-oiled machine like Verisign, which operates the .com and .net TLDs, among others. As a result, sometimes with a less reliable registry, there can be false positives, resulting in the registrar showing a domain as "available" when it is actually registered.
In addition to registry connection reliability, there are also many different aftermarket sources that registrars often pull from. You know when you see a Premium domain (registered and usually higher priced) in search? That could be coming from any number of 3rd party aftermarket platforms, which also can have varying reliability and/or stale listings.
Lastly, you have to consider that some registrars handle the "drop window" differently than others. If a domain deletes and is removed from the zone, ergo, becoming available again, some registrars have a buffer period before they show it as available again.
