There is no single app subscription for Lightroom Classic because the 20GB Photography Plan is already excellent value. Many of Adobe's single app subscriptions are around US$20.99 per month - that's the cost of Illustrator, InDesign, Premiere Pro, After Effects and Photoshop (which is bundled with Fresco). Getting Lightroom Classic for US$9.99 per month is already good value even if you don't use anything else in the Photography Plan bundle.
When you compare the cost to the update subscription for Topaz's photo products (US$99 per year for all their photo plugins that you own) or the cost of a version upgrade for DxO PureRaw (US$79, I believe), both of which require you to already own the products, it tends to indicate that Adobe's pricing is competitive. Of course, if you stop paying for the Photography Plan, then the Develop, Map and sync features stop working.