Indeed. No guarantee.
Apparently last Monday Twitter tried to shut down third-party API access even more, but failed and cut off its own clients (and website!) as well… 
However, I’m afraid they will keep trying and succeed sooner or later 
If I had any assurance that I could keep using Spring for Twitter (even for a few months) I would buy it right now, to follow the minority of people/companies that have not moved to Mastodon (yet?).
My Spring app is not working but Spring Beta from Test Flight is working.
With your own API key?
One trick that works for me on desktop is keeping bookmarks to the specific lists on Safari. Perhaps it can be automated with the iOS client to have shortcuts on the iPhone.
I have moved on (my Twitter usage consisted of mostly reading tweets from the tech community - and the tech community has pretty much moved over to Mastodon). I do not miss a thing to be honest.
Regarding Twitter and its API - it apparently is coming back (?):
Paul Haddad:
We never had any kind of analytics to get a better # but my gut says a Twitter client would probably download 100k-250k Tweets/month per account per device. So if we get about 200 people willing to pay $400-500/month we could come back!
That sounds like a bargain, I guess. 
(
Sarcasm alert…)
In other news:
In theory this could mean that “Facebook” users could interact with Mastodon users in the long run. Or not.
No matter what: a decentralized network is the way to go in the future, it is a necessity and it is feasible. The cat is out of the bag.
I’m not the biggest Facebook fan, but a Twitter-killer could be what the federated space needs to bloom.
Yes. Working right now.
Apparently you need a key with (v2) “Elevated Access”.
I only have a v1.1 key from an App I made (but never released) years ago.
Is it hard to get this elevated access?
Nope, took about a minute of filling a form in.
without any application review process, you’ll be approved
That sounds promising. Perhaps I should give this a try?
(OTOH the friction added by the App I currently use might help me stay away from Twitter more…)
Not my experience; I requested Elevated Access and am waiting for their review (for several days now).
How long does this take? (Anyone already went through this?)
Still no reply from Twitter, but the author of Spring informed me that it still works out of the box, which it does! 
With my usual “luck” I think that €17,99 will be wasted very soon, so apologies if Twitter finally shuts down third-party API access now…
Thanks for these updates. I applied for the API as well and am still waiting to hear back. Good to know that the app works without it and that I can spend $17 for 3 days of Twitter use before they shut the app down on us!