Is it 'ok' to have IFrame tags in RSS feeds?
Well, as the error states, "interoperability with the widest range of feed readers could be improved" ...if the
iframe tag was removed.
This is understandable since the main purpose of the
iframe tag is to import external content from another site - this could literally be anything. Why would you want/need to do this in an RSS feed? So, from a security point of view, RSS clients might block it. What is this
iframe doing in the RSS feed? Any "content" should be embedded directly in the RSS feed, not dynamically inserted from another site - if that is what you are doing.
Is it harmless enough to leave as is or do I have to take it out?
It might be harmless for your site, but as stated, some RSS feed readers may simply strip it out - so users won't see the content.
what I would replace it with if I took it out.
Whatever content the
iframe is importing should be embedded directly in the RSS feed.
UPDATE (from comments):
This RSS feed is being creating by a WordPress blog page and on some of the blog posts we have YouTube videos.
There's not much you can do about embedding YouTube videos. To be honest, most(?) users will probably see it OK (Feedly certainly supports iframed YouTube videos) - but there maybe some users that won't. That is really all the "warning" you are seeing is about - making your content accessible to the largest possible audience. For these (few) users they should be able to follow a link to view the full post/video if they wish.