Android O's fourth dev preview has a Paste As Plain Text option that appears when you go to paste something you've copied that has any sort of formatting included. This is something which was recently discovered as part of the fourth and final Developer Preview for Android O, so there's a good chance that it will transition into the stable, public build that Google intends to send out to users in the near future, though it's worth keeping in mind that nothing is official until everything is all said and done. Since there is still likely a month at least before Android O's final build heads out to users, Google could always decide to change things and remove this option before pushing out the software update.
For those that are on the Android O Developer Preview 4 software, trying this is simple enough, but the option to paste something as plain text will only show up if what was copied has formatting, which means if you want to check out the feature for yourself you'll need to find something that has formatting, copy it, then go to paste it somewhere, such as in a text or a note.
Paste as plain text is part of Android O's smart text selection changes, which also includes the ability to go to whatever application would make the best use of things like phone numbers or addresses when those things are copied. For instance, if you're copying a phone number, the smart text selection bar would include an option to go to the phone app and automatically populate the number, allowing you to place a phone call to it pretty quickly. It's reported that this paste as plain text option is available across the system meaning you should be able to see it in just about any application so long as there is something with formatting to copy. While not all users will be able to make good use of this option, if you frequently work from your phone for any sort of task like typing up documents or emails, it can help you avoid having to reformat stuff, or give you a clean slate to work with if you want to insert formatting but do it in your own way instead of having to change from whatever it was in the copied content.