Using an authentication provider to keep a user permanently logged in eventually causes session problems Because I got fed up of having to log in to my local development sites every however long, I tried making a simple custom module that just has an Authentication Provider service that always returns user 1, like this:
  /**
   * {@inheritdoc}
   */
  public function applies(Request $request) {
    return TRUE;
  }

  /**
   * {@inheritdoc}
   */
  public function authenticate(Request $request) {
    return $this-&gt;entityTypeManager-&gt;getStorage('user')-&gt;load(1);
  }

The idea being that with this module enabled, every page load considers the authenticated user to be user 1. In theory, I'd be logged in for ever!
However, after a few weeks of this working fine, all my form submissions broke! Every form submission fails with:

The form has become outdated. Press the back button, copy any unsaved work in the form, and then reload the page

This turns out to be because CsrfTokenGenerator::validate() fails. Specifically, the $seed in this bit is just NULL:
  public function validate($token, $value = '') {
    $seed = $this-&gt;sessionMetadata-&gt;getCsrfTokenSeed();
    if (empty($seed)) {
      return FALSE;
    }

I presume it's because the session has expired in some way? The time this has taken to stop working is roughly the amount of time it would have taken the site to log me out automatically.
How can I fix this?
I presume it's because the session has expired in some way?

First you need to understand how this happens.

Configuring Garbage Collection
When a session opens, PHP will call the gc handler randomly
according to the probability set by session.gc_probability /
session.gc_divisor. For example if these were set to 5/100
respectively, it would mean a probability of 5%. Similarly, 3/4 would
mean a 3 in 4 chance of being called, i.e. 75%.
If the garbage collection handler is invoked, PHP will pass the value
stored in the php.ini directive session.gc_maxlifetime. The
meaning in this context is that any stored session that was saved more
than gc_maxlifetime ago should be deleted. This allows one to expire
records based on idle time.
However, some operating systems (e.g. Debian) do their own session
handling and set the session.gc_probability variable to 0 to stop
PHP doing garbage collection. That's why Symfony now overwrites this
value to 1.
If you wish to use the original value set in your php.ini, add the
following configuration:
# config/packages/framework.yaml
framework:
    session:
        gc_probability: null

Source: Symphony's official docs on Configuring Sessions and Save Handlers

In Drupal 8 &amp; 9 you can see these gc settings in default.services.yml
Possible Solutions on how to stay logged in longer.
In /sites/default/settings.php uncomment
 if (file_exists($app_root . '/' . $site_path . '/settings.local.php')) {
   include $app_root . '/' . $site_path . '/settings.local.php';
 }

then copy /sites/example.settings.local.php and rename to /sites/default/settings.local.php
Now you can use /sites/development.services.yml to override those settings, for example
(I used these numbers for testing so it logs me out after 2 seconds):
parameters:
  session.storage.options:
    gc_probability: 1
    gc_divisor: 1
    gc_maxlifetime: 2
    cookie_lifetime: 2

I tried setting gc_probability to 0 or null, which should stop it, but the gc() function still fired. Perhaps it's reading the value from php.ini
So alternatively you could set the gc_maxlifetime and cookie_lifetime to not expire decades from now. See set a cookie to never expire.