Many projects have these „project-local“ or „company-local“ libraries, that can be used optionally or are even strongly imposed on developers. They may be called something like
or even with a meaningful good name.
Generally there is nothing wrong with such libraries, if they are used correctly.
Some things should be observed, though: Such libraries are there to be useful. If that is not the case, it is better not to use them and to deprecate and discard them. Quite a lot of projects suffer from the fact that inferior local libraries are imposed on them and have to be used. The strive for „consistency“ is not bad either, but it should be kept to a level at which it is useful and not become a primary goal by itself.
So when need for a functionality arises, an existing library might cover this well enough. If it is not too hard to integrate, there is little need to write a „local library functionality“ for this. If no good library can be found or reasonably be integrated, it is a good idea to write what is needed oneself. It should be observed, that such a library should have slightly higher quality standards and very good automated testing, because it is more universal and the actual and future usage cannot be anticipated as easily as with „business code“. There are places, where it is good to impose a local library, to make sure that certain fields are validated in the same way across the software or even across the organization, for example. For data like phone numbers, dates, email addresses etc. that are commonly used in our world, it is probably possible to find good libraries that do this much better than any local library with reasonable team effort could do.
Now the world moves and we might discover better replacements for parts of the local library. It is a good idea to move on to these better replacements.
Things that can go into a local library are small pieces of business logic that need to be used everywhere. Think of an insurance company. Customers have customer numbers. They have to be entered into web application forms and they have to be checked for correctness, they have to be formatted etc. This could go into a library and we could be sure that everyone in the larger project uses the same rules for what is a valid customer number and how to format and parse them. It is a bit too hard to implement it again and again, because tiny errors sneak in, for example into the code to check for validity with some check sum, but it is too easy to put it into a service that can be called to check formal validity. Such a service will be there to check if some formally correct customer number is actually a real customer’s number, maybe with constraints on who can see this for which customer. And of course to generate new customer numbers, when needed.
It is always good to ask twice if certain functionality should rather sit in a service that can be called or in a library, of course depending also on the local architecture preferences. In cases where the functionality is useful for others in the organization, but where the exact same behavior is less important, it can also be an option to just copy the code to different teams. But this should be used with care, because it means that improvements do not easily flow back to benefit other users of the functionality.
Generally it is an observation that organizations rarely have good „local libraries“. The good libraries are found on the internet. Or only exist within the team. And the bad company libraries are often forced on those who cannot find ways to opt out. Good team libraries that are not known outside of the team can sometimes be a loss for the organization, because things are done twice or even worse are done differently where it would be good to have the same behavior.