Create Podcast Seasons in Seriously Simple Podcasting
To separate your podcast into disctinct Seasons you can use the iOS11 custom podcast fields within Seriously Simple Podcasting.
When you originally install the plugin these options are off, to keep things clean in the post editor and because everyone doesn't need to use them, but you can turn them on for all episodes in the Podcast - Settings - General area of your wp-admin dashboard.
With this setting enabled when you view/edit a podcast post you will see these additional fields which give each episode a bit more information, including a Season Number.
Fill in as many of these fields as you would like, but most importantly for if you're wanting to create distinct Seasons for your show this is where you would enter the "iTunes Season Number" value.
Assign each episode a Season Number and update the posts. This will add additional information to your RSS feed which Apple Podcasts will then display to your audience as Season Number.