I was reading that WordPress discontinued Post Formats in version 3.6.
It is about Post Formats UI. This link is valuable for you,
Does that mean that even though they are listed when writing a post, they are not currently usable?
Yes they are listed and available in core.
They won’t have any effect?
Post formats is all about Content Presentation e.g. Video posts will display in different manner as compared to Quote, Audio etc posts. Please visit the Contango Demo to understand the concepts of Post Formats.
If that’s true, should I choose Standard Format for all posts for now, or use the appropriate Post Format so that it ‘activates’ in the future when WordPress brings them back?
You can use Post Formats because WordPress will definitely provide the alternate solution for her users in case of dropping Post Formats as a core feature. e.g. in the shape of Post Formats Plugin.