I still don't understand why about this note:  https://releasenotes.docs.salesforce.com/en-us/spring18/release-notes/rn_networks_hashtags_thanks_polls.htm

1) at my understanding the hashtag is available in winter 18 in the Thanks Action in Classic.

2) There is no way to Add the Thanks action to the chatter.  I tried adding the "Thanks" Lightning action to the publisher layout, to the objects page layout and this action its not showing.  So Im not able to see this hashtag feature in LEX spring 18.