To conform with the Wikimedia principle that everyone is allowed to contribute without creating accounts, Wikimedia Space should allow IP participation.
Presumably this refers to the founding principles, which say "almost anyone [should be able] to edit (most) articles without registration". I don't think that automatically extends to discussion spaces (most of which already require an account of some sort - e.g. you need to register an email address to participate on the mailing lists). Articles are seen as the point of entry for new contributors, so keeping the barrier to entry as low as possible helps (and you don't need any familiarity with the community or movement to spot a spelling error). If the Wikimedia Space is similarly seen as a contact surface with people completely new to the movement, keeping the barrier of entry as low as possible is important. If it's mainly seen as a communication space for people who are already in the movement, then SUL login should suffice. Those both seem like potentially valid approaches to me.
What @Tgr said. We are discussing this request in Phabricator, where anonymous contributions aren't allowed either.
Besides, the possibility to post without registration exposes us to spam, vandalism and harassment for no real benefit. If someone wants to post anonymously, they can get a free email address somewhere, create a Wikimedia account with some text string as username, and from there get a Space account.
All this applies to articles as well, if you assume that unregistered users don't have what to contribute. Requiring registration is an obstacle to participation. Village pumps and other community discussion areas are open to contribution by all.
if you assume that unregistered users don't have what to contribute
What we assume is that the problems related with opening participation to unregistered users outweigh the potential benefits.
The Wikimedia Space team is unanimous on the decision of offering a read access for everyone, and participation featured to registered users only. It is an intrinsic part of this proposal. For this reason, this request is being declined.