I use the HTML5 cache manifest for the offline mod of my web app but I saw that this feature will be removed of the HTML5 standard..
The HTML Standard says this:
This feature is in the process of being removed from the Web platform. (This is a long process that takes many years.) Using any of the offline Web application features at this time is highly discouraged. Use service workers instead.
Someone has more info about it? Is it safe to continue my web app with this feature?
Browsers still support the offline Web applications (cache manifest) but browser vendors are not interested in continuing to support it forever. They plan to remove support after a better mechanism is added to the platform.
That said, as indicated in the note you cite from the HTML Standard, it will be years before browsers are able to remove support for cache manifests. So it’s possible for you to continue using the feature, though as the note indicates, doing that is highly discouraged.
The alternative to cache manifests that you are encouraged to use instead is Service Worker—in particular, the Service Worker Cache and CacheStorage interfaces. Those provide a much more robust solution for offline Web applications.
The cache-manifest feature, in contrast, has many problems and limitations—among them the fact that it can be very hard for you to troubleshoot problems with it when they occur.