It is the end of the road for the HTC One M7 as it will not receive further updates for the latest Android 5.1 Lollipop. The latest software version for the original One would be the initial Android 5.0 update which is being rolled out right now.
The HTC One M7 has been around for about 2 years and it has gone through several stages of updates from its default Android 4.1.2 Jelly Bean. Considering Android 5.0 is an early Lollipop release, fans weren’t too happy that HTC is leaving them in a lurch with initial bugs and rough edges that would be addressed in the incremental Android 5.1 update.
The only chance to get Android 5.1 officially would be the HTC One M7 Google Play Edition which is targeted for April. If your HTC One M7 is on Android 4.4.3 with no problems, you might want to reconsider downloadig the next software update that comes in. Hopefully HTC would reconsider continuing its updates to at least Android 5.1