To the best of my knowledge (and IANAL) a good example of this is the European Court of Justice Case C-355/12 "Nintendo v. PC Box" where europas highest court ruled that DRM must respect the principle of proportionality and circumvention of technical protection is only illegal if done for unlawful purposes. If DRM prevents lawful purposes, then the DRM is not proportional to consumer rights
(or those of other corporations). If circumventing DRM is illegal is a question of the specific case and must take into account the purpose of the tech used to do it and what people actually use it for.
This, at the core, means it is very hard to argue that some library written to circumvent DRM is "illegal tech" and have it taken down in this manner, because the DRM could be inappropriate and it is not the copyright owner who has to decide that.
The ECJ was asked by a Milan court for a preliminary, so they gave instructions how the Milan court should handle the matter and how the law is to be interpreted. The base case was about a mod-chip sold by pc-box, who argued as a defendant, that circumventing nintendos DRM for the purpose of playing homebrews was ok and nintendo preventing that is inappropriate.
In the general case this was a huge win, because "circumventing DRM is illegal" is only true with a big IF, not as a blanket statement. And from that follows that usage of alternative clients is well within the consumers rights, but again i am not a lawyer.
However note that the Milan court then ruled in its case 12508/2015 that this particular mod chip is illegal. Nintendo gave a lot of evidence about the advantages of their DRM in terms of cost, ease of use, security etc, comparing it to inferior implementations of their choice that would fail to protect the copyright holders interests, as well as evidence of usage of the mod chip for piracy. On the other side pc-box defaulted, filing no evidence showing that their users are a vibrant community of homebrew gamers and techheads that circumvent DRM for purposes well within their rights, like running self written software on the hardware. The Milan court also argues that defendant has a burden of proof to show that a more proportionate drm method was possible, which i strongly disagree with, and which seems to follow nintendos argument that their solution is appropriate even if more restrictive then strictly necessary. Note that Milan does not speak for the EU.