What is “Low Data Mode” on my iPad’s connection to my iPhone as hotspot? I have my iPhone configured to act as a hotspot, providing internet access via cell data.
When connecting my iPad as a WiFi client to that hotspot, the Settings panel notes “Low Data Mode” below the name of the connected hotspot. Indeed, while using the iPad, the internet connection seems much slower than on the iPhone.
My carrier, T-Mobile, does limit the amount of fast data delivered via “tethering”. Is this “Low Data Mode” note a sign of that limit being reached?
I have never not seen that subtitle, so I am left uncertain.
According to this source Apple introduced a new feature to reduce and limit cellular and Wi-Fi usage. This feature is called Low Data Mode. You can use this feature on your iPhone, iPad or iPod touch to limit the mobile data used in background mode by apps.

Different apps use different ways of reducing data usage in Low Data
Mode. In general, here's what you can expect:
Apps might stop using
network data when you're not actively using them.
Background App
Refresh is turned off.
The quality of streaming content might be
reduced.
Automatic downloads and backups are turned off.
Services such
as iCloud Photos pause updates.
Built-in iOS apps and services adapt
to Low Data Mode in the following ways:
App Store: Video autoplay,
automatic updates, and automatic downloads are turned off.
Music:
Automatic downloads and high quality streaming are turned off.
Podcasts: The frequency of feed updates is limited, and episodes are
downloaded only on Wi-Fi.
News: Article prefetching is turned off.
iCloud: Updates are paused, and automatic backups and iCloud Photos
updates are turned off. FaceTime: Video bitrate is optimized for lower
bandwidth.

See Apple support page, Use Low Data Mode on iPhone, iPad, and iPod touch.
Please note that Low Data Mode is automatically enabled when you are connected to a Personal Hotspot.