Side effects of Android CarrierConfig reporting a foreign network as ”not roaming” Android has a system app, CarrierConfig, which sets certain network parameters based on the carrier which supplied the currently inserted SIM card. Among others, the CarrierConfig app can tell Android that certain networks (identified by the MCC/MNC pair) should not be considered roaming even though the MCC/MNC differs from that on the SIM. (See this answer to a previous question of mine for details on its inner workings.)
I am wondering if this mechanism can be used for EU roaming. As of last year, EU carriers can no longer charge roaming fees for use on other EU networks, as long as the majority of usage is in the “home state” of the SIM card, or the subscriber can demonstrate some ties with that state (e.g. residence, work, study or family).
The idea would then be: for every EU carrier, insert a list of all EU MCCs into its list of non-roaming networks (omitting the MNC makes it a catch-all for every network with that MCC).
As I understand the mechanism, the effect would be that, with a SIM from an EU carrier, the phone would then assume to be on its home network. Roaming restrictions (such as disabling mobile data) would then only apply outside the EU. For example, if I disabled data roaming, my phone would still have data connectivity in Austria, France and Italy but not in Switzerland.
I suggested this to the LineageOS folks the other day and was told this was not going to happen, as in their opinion EU roaming is still roaming and such a rule would break it. Unfortunately they did not elaborate on that, so I don’t know if there’s some undesirable side effect I’m missing.
The only side effect of the “EU-to-EU = not roaming” ruleset that I can see is that the user can no longer make settings à la “use data services on network 232-07 only, but not on other networks even in the EU”. Whether this constitutes a relevant use case is debatable.
Is there anything else I have missed (i.e. functionality this change will break)?
I can not add a comment in your question because I have not the neccesary score to do it. So, the only way is doing a new answer 
As I explain you in the other question you could add to any network, any other network of other country for that it do not behave like a roaming.
I am going to explain my point of view about the problem to develop this feature. It is true that although roaming in europe is free depends on some factors as you said, for example:

As of last year, EU carriers can no longer charge roaming fees for use on other EU networks, as long as the majority of usage is in the “home state” of the SIM card, or the subscriber can demonstrate some ties with that state (e.g. residence, work, study or family).

If LineageOs develop this feature and for example you have a French SIM and you live in Italy, there will come a time when the French operator will start charging a plus for the roaming.
And it could be a problem, because if the user have not activated data roaming, the user won't know that the operator could do that.
The user need to know that it is possible that the roaming may produce an extra cost on your bill.
It is possible that you has a French SIM because you are french, and you live in Italy because your work is in Italy, and you could think, &quot;Ok, the European single market should allow me to use the same SIM in different countries at no cost&quot; and that was the original idea of the end of roaming.
But some operators (the big carriers) complained that people could buy SIMs in countries with cheaper rates to use them in countries with higher rates.
So, summarizing, I think that it is possible the feature that you want (technically talking) but it is not recommended.