For what it's worth, this is breaking an API client to WooCommerce for us.

With 5.4.20

> GET /wp-json/wc/v3/orders?status%5B0%5D=processing&status%5B1%5D=completed&after=2023-03-29T10%3A35%3A09%2B02%3A00&before=2023-03-29T11%3A04%3A18%2B02%3A00 HTTP/2
Host: gazon-naturel-en-rouleau.fr
pragma: No-cache
accept: */*
authorization: Basic **********
user-agent: Symfony HttpClient/Curl
accept-encoding: gzip

* old SSL session ID is stale, removing
* Connection state changed (MAX_CONCURRENT_STREAMS == 100)!
< HTTP/2 200

With 5.4.21

> GET /wp-json/wc/v3/orders?status[0]=processing&status[1]=completed&after=2023-03-29T10:35:09+02:00&before=2023-03-29T11:02:48+02:00 HTTP/2
Host: gazon-naturel-en-rouleau.fr
pragma: No-cache
accept: */*
authorization: Basic *************
user-agent: Symfony HttpClient/Curl
accept-encoding: gzip

* old SSL session ID is stale, removing
* Connection state changed (MAX_CONCURRENT_STREAMS == 100)!
< HTTP/2 400

This is how the request is constructed:

$response = $client->request(
    'GET',
    $this->currentShop['endpoint'],
    [
        'auth_basic' => [
            $this->currentShop['api_key'],
            $this->currentShop['api_secret']
        ],
        'headers' => [
            'Pragma' => 'No-cache',
        ],
        'query' => [
            'status' => ['processing', 'completed'],
            'after' => $dates['after']->modify('-5 minutes')->format('c'),
            'before' => $dates['before']->modify('-5 minutes')->format('c'),
        ],
        'on_progress' => function () use ($downloadProgress): void {
            $downloadProgress->advance();
        },
    ]
);

Which part/character? You can know by generating the query string on your own to test.

It's the plus signs here.
Guessing the problem is that un-encoded plus signs stand for space, but here, they stand for timezone offsets. So they must be encoded to plus signs, not spaces.

OK, so this is #49579, fixed in the next release.

This has broken our aplication should not we stick confrontant to RFC?

@Kozzi11 Please open a new issue if you think this introduced a bug.
Also note that unlike your application, Symfony is mostly non-profit work so please use a more relaxed language.

@Kozzi11 Please open a new issue if you think this introduced a bug. Also note that unlike your application, Symfony is mostly non-profit work so please use a more relaxed language.

@chalasr Yeah you are right, I appologize for that. I have created Issue #50670 and PR #50671