Query by multiple custom taxonomies

Based on your code, if the url is:

http://example.com/page/?eth=1&t=1

Then the resulting query will be:

$tax_query_args = array(
    'relation'      => 'OR',
    array(
        'taxonomy' => 'filters',
        'field'    => 'slug',
        'terms'    => array( '', 'twitter', '', '', '', '', '', '', '', '' ),
        'operator' => 'IN'
    ), 
    array(
        'taxonomy' => 'platform',
        'field'    => 'slug',
        'terms'    => array ( 'ethereum', '', '', '', '', ''),
        'operator' => 'IN'
    ), 
);

While the empty variables are a bit messy, WordPress will ignore them. So really you’re just querying posts that have the ‘twitter’ filter or ‘ethereum’ platform. So if that’s what you want, the code is fine.

If it’s not working then either the slugs or taxonomy names are incorrect, or there’s something else wrong with the query that you are using $tax_query_args in. It’s impossible to say without seeing more, but what you have here is fine.

With all that said, I want to suggest a considerably cleaner way to approach this.

You mentioned checkboxes, and based on your use of $_GET I’m assuming your form looks like this:

<input name="t" value="1" type="checkbox">
<input name="f" value="1" type="checkbox">
<input name="eth" value="1" type="checkbox">
<input name="neo" value="1" type="checkbox">

Doing it this way means you need that huge block of code just to figure out which taxonomy terms were selected.

I suggest you change the form to work like this:

<input name="filter[]" value="twitter" type="checkbox">
<input name="filter[]" value="facebook" type="checkbox">
<input name="platform[]" value="ethereum" type="checkbox">
<input name="platform[]" value="neo" type="checkbox">

Now all you need to do to query the right taxonomy terms is:

$filters = isset( $_GET['filters'] ) ? $_GET['filters'] : array();
$platform = isset( $_GET['platform'] ) ? $_GET['platform'] : array();

$tax_query_args = array(
    'relation'      => 'OR',
    array(
        'taxonomy' => 'filters',
        'field'    => 'slug',
        'terms'    => $filters,
        'operator' => 'IN'
    ), 
    array(
        'taxonomy' => 'platform',
        'field'    => 'slug',
        'terms'    => $platform,
        'operator' => 'IN'
    ), 
);