IndexSet.cs

public ref struct IndexSet
{
	public static int SizeInBytes(uint maxIndex)
	{
		var indicesCount = maxIndex + 1;
		var (size, additionalBits) = ((int)(indicesCount >> 3), indicesCount & 0b111);
		if (additionalBits > 0)
		{
			size++;
		}

		return size;
	}

	private readonly Span<byte> _bits;
	private readonly uint _maxIndex;
	private int _count;
	public int Count => _count;

	public IndexSet(uint maxIndex, Span<byte> bits)
	{
		var bitsCount = bits.Length << 3;
		if (maxIndex >= bitsCount )
		{
			throw new ArgumentException($"Bits count '{bitsCount}' is not enough to cover range of indices: [0; {maxIndex}].");
		}

		_maxIndex = maxIndex;
		_bits = bits;
		_count = 0;
	}

	public bool Add(uint index)
	{
		if (index > _maxIndex)
		{
			throw new ArgumentException("Value is greater than max index.", nameof(index));
		}
		var (@byte, bit) = ((int)(index >> 3), (int)index & 0b111);
		var @new = (_bits[@byte] >> bit & 1) == 0;
		if (@new)
		{
			_bits[@byte] |= (byte)(1 << bit);
			_count++;
		}
		return @new;
	}

	public bool Remove(uint index)
	{
		if (index > _maxIndex)
		{
			throw new ArgumentException("Value is greater than max index.", nameof(index));
		}
		var (@byte, bit) = ((int)(index >> 3), (int)index & 0b111);
		var existed = (_bits[@byte] >> bit & 1) == 1;
		if (existed)
		{
			_bits[@byte] &= (byte)~(1 << bit);
			_count--;
		}
		return existed;
	}

	public readonly bool Contains(uint index)
	{
		if (index > _maxIndex)
		{
			throw new ArgumentException("Value is greater than max index.", nameof(index));
		}
		var (@byte, bit) = ((int)(index >> 3), (int)index & 0b111);
		return (_bits[@byte] >> bit & 1) == 1;
	}

	public uint[] Indices()
	{
		var items = new uint[_count];
		var added = 0;

		for (var index = 0U; index <= _maxIndex; index++)
		{
			var (@byte, bit) = ((int)(index >> 3), (int)index & 0b111);
			var exists = (_bits[@byte] >> bit & 1) == 1;
			if (exists)
			{
				items[added++] = index;
			}
		}

		return items;
	}
}

There's similar System.Collections.BitArray