This is by no means an official support statement from Microsoft, rather an unofficial compilation of official statements.
Last week the SharePoint Online team rolled out the preview of the Modern SharePoint lists. Modern Lists are the new incarnation of ye ole Classic SharePoint lists that we all loved and hated over the last decade or so. The Classic SharePoint lists and libraries has been one amazing and powerful tool and I would say that they have been a big part of the success SharePoint has had. Customizations using XSLT, SharePoint Designer and JSLink has all contributed to its success.
The new Modern Lists (currently in preview, not even in First Release and you need special voodoo tricks to get it to work) does not support these kind of customizations, for now. Over the last week or so I've heard large cries and worried Tweets about all peoples current customizations. Microsoft has not been overly clear on the supportability for classic lists in public writing but rest assured; Classic lists will be here as long as they are used and needed. This is what I've been told by the team and what they are trying to say;
- "We have no plans to remove classic mode anytime soon"
- "We heard your feedback on extensibility and customization in particular, and we’ll have more to share in a future update. We plan to add support for customizing the page using modern techniques. Until then, customized library pages should stay in classic mode."
- "Classic mode supports your customizations today, and tomorrow."
- "In the meantime, it’s important for us to maintain continuity for our existing customers. "
Also I do think it is good that they don't set a date for supportability, which some people request, like they've done with for instance InfoPath. If they do that, they have to support Classic Lists until that date. I think it is better they make great progress in making us move to Modern Lists as soon as possible, and when they see no or minimal usage of Classic Lists, they can remove it. The same as they've done for Sandboxed Solutions - something we known coming for years! [Update] But yes, I do think a notification a little bit more than 30 days prior to shutting it down makes sense, if and when the feature gap is closed.
So, keep calm and continue to use your classic lists - however start planning for a modernization of your XSLT List View Web Parts, your JSLink customizations and more. I know that what's coming to replace these, will be much more interesting and be way more modern.