After managing to take down IMSLP.org with no warning, the Music Publishers Association (UK) has now begun to cooperate with IMSLP. The website is back online, and MPA is publicly communicating their regrets and intentions to do better next time.

Read more about the situation on the IMSLP forums:

I wish to thank the Music Publishers Association of the UK for retracting their DMCA complaint to GoDaddy. GoDaddy's standard response to a DMCA copyright complaint of this nature is to freeze the domain for 10 days, instead of referring the complaint to the site owner. Thus we all owe the MPA-UK sincere thanks for their retraction.

Screenshots from the MPA's Twitter feed.