Is it bad practice NOT to delete unused database entities? A recent code review by a client revealed that there some database entities (tables, views, stored procedures) were no longer being used in our Oracle 12c database. We have hundreds of stored procedures, views and tables (not thousands), and the database itself is about 40GB in size with a few hundred million records. I don't know the number of unused entities but I'm told it would take 4 hours or so to remediate.

The problem is that regression testing would take a huge amount of time.

I've been told that having unused entities is common and is not necessarily bad as they could be used at some future time. But on the other hand, this can be confusing to others who are working on the system. 

I'd like to have a better perspective on this -- can this be considered a fault developers need to remediate, or is it a non-issue?

Thanks
In case of tables, if it's not being updated you are safe to remove then and keep a backup (just in case).

Other objects like SP, Views and Functions can be easily removed.

Of course you can end up messing with relational integrity if you are not cautions in removing data. I recommend full regression tests over the system.

The advantages of removing unused objects is keeping the schema (and the system) clean and simple, this helps maintenance.

The disvantage is the risk of lost of essential data, damaging relational integrity and jeopardizing the system.

Also be aware if you remove a substancial mass of data it smells like trouble.

Edit

This is long comment full of "IMHO".
I'm a developer happened to get some dba tasks by accident. When they say "it's better let things as they are" it means "ok, cleaning the schema is fine but developers and testers will get they hands full of (unnecessary?) work and they are already full"

And that's true, in special if there acumulating garbage from years of "better let things as they are". Developers can need to remove references to that superflous objects. Testers must testify nothing ill blow up, etc.

At the end of the day someone needs to make the hard decision: To clean or not to clean, that's the question. If you can (do you have good documentation? have the time? Can alloc coworkers?) make a "possible impacts" report to your manager where you can pontuate the advantages, disvantages, risks and answer one little question: Are us just postponing a job needs to be done some point in the future?
Btw the short answer for your quastion is Yes!